Effect of hydrophilic group on water droplet contact angles on surfaces of acid modified SiLK and Parylene polymers

Abstract

The effect of surface roughness and surface chemistry of SiLK and Parylene polymers on the water droplet contact angle was investigated using Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). AFM images show that the surface morphologies of SiLK and Parylene treated with H 2SO 4 and H 3PO 4 solutions are qualitatively the same as those of the untreated SiLK and Parylene. However, the advancing ( θ a) and receding ( θ r) contact angles on SiLK dramatically decrease from θ a = 70 ± 3° and θ r = 63 ± 2° to θ a = 37 ± 2° and θ r = 34 ± 3°, respectively, after being treated with a H 2SO 4 solution. In contrast, the advancing and receding contact angles on SiLK treated with H 3PO 4 solution or Parylene treated with H 2SO 4 or H 3PO 4 solutions were changed within a few degrees. XPS results show that polar functional groups (C O and –OH) formed on the SiLK sample surface after being treated with H 2SO 4 solution. However, H 3PO 4 reacts less aggressively with the ethynyl (–C C–) bonding on the SiLK surface compared with H 2SO 4. Parylene lacks ethynyl chemistry and is therefore chemically more inert than SiLK. Thus, both H 2SO 4 and H 3PO 4 solutions have fewer reactions with Parylene resulting in slight changes in the advancing and receding contact angles.